1. Field of the Invention
The present invention relates to a beam delivery system for use with lasers, and particularly for use with discharge pumped molecular fluorine lasers emitting around 157 nm.
2. Discussion of the Related Art
Molecular fluorine (F.sub.2) lasers operating at a wavelength of approximately 157 nm are a likely choice for deep UV/ vacuum UV microlithography with resolution below 0.1 micrometer. Laser radiation at this wavelength is also very useful for micromachining applications involving materials normally transparent at commonly available laser wavelengths.
Efficient extracavity transport of a sub-200 nm laser beam to the target is complicated by strong absorption in the atmosphere. That is, the sub-200 nm laser beam of such a laser will propagate a certain distance along an extracavity beam path between the laser output coupler and a work piece where it is subject to absorptive losses due to any photoabsorbing species such as water, oxygen and hydrocarbons located along the beam path. For example, an extinction length (1/e) for 157 nm radiation emitted by the F.sub.2 -laser is less than a millimeter in ambient air.
High intracavity losses also occur for lasers operating at wavelengths below 200 nm, again due particularly to characteristic absorption by oxygen and water, but also due to scattering in gases and all optical elements. As with the absorption, the short wavelength (less than 200 nm) is responsible for high scattering losses due to the wavelength dependence of the photon scattering cross section.
These complications from absorption and scattering are much less of a problem for conventional lithography systems employing 248 nm light, such as is emitted by the KrF-excimer laser. Species such as oxygen and water in the cavity and atmosphere which absorb strongly below 200 nm, and specifically very strongly around 157 nm for the F.sub.2 laser, exhibit negligible absorption at 248 nm. The extinction length in ambient air for 248 nm light is substantially more than ten meters. Also, photon scattering in gases and optical elements is reduced at 248 nm compared with that occurring at shorter wavelengths. In addition, output beam characteristics are more sensitive to temperature-induced variations effecting the production of smaller structures lithographically at short wavelengths such as 157 nm, than those for longer wavelength lithography at 248 nm. Clearly, KrF excimer lasers do not have the same level of problems since the 248 nm light scatters less and experiences less absorption.
One possible solution for dealing with the absorption problems of the 157 nm emission of the F.sub.2 laser is sealing the beam path with a housing or enclosure and purging the beam path with an inert gas. However, high flow rates are typically used in this technique in order to minimize the down time needed to remove absorbing species from the beam enclosure. That is, starting from a state where the enclosure is filled with ambient air, an unacceptably long purge time and high flow rate would be required to bring the partial pressure of absorbing species down to a reasonable level. It may also be necessary to perform this purging technique with a very clean inert gas, e.g., containing less than 1 ppm of absorbing species such as water and oxygen. Commercial ultra high purity (UHP) grade gases may be obtained to satisfy these purity requirements at increased cost. Overall, this purging approach is expensive and inconvenient.
Another solution would be evacuating the beam path. In this case, a relatively low pressure vacuum would be needed resulting in a complex and expensive system. For example, ultrahigh vacuum (UHV) equipment and techniques may be necessary for achieving a pressure below 100 millitorr. Such equipment and techniques combine a tight enclosure with high pumping capacity. Unsatisfactorily long initial pumping times would still be required. In this evacuation approach, transmission along the optical beam path enclosure would be determined by the absorption of radiation by "residual" gases, mainly oxygen, water vapor and hydrocarbons which remain despite the evacuation.
FIG. 1 shows an experimentally measured dependence of the transmission of a 0.5 meter optical path on the residual air pressure. A theoretical fit is also shown in FIG. 1 and is based on the assumption that the main absorbing species is water vapor having an absorption cross-section of approximately 3.times.10.sup.-18 cm.sup.2. This assumption is believed to be justified because water has a tendency to be adsorbed at the walls of vacuum systems and thus, to dominate the residual pressure in such systems.
As can be seen, at a residual pressure of 50 milliTorr, the optical losses amount to about 1% per each 0.5 meter of the optical path. At around 100 milliTorr, the optical losses amount to about 2% per each 0.5 meter. At 150 milliTorr and 200 milliTorr, respectively, the losses amount to 3% and 4.5%. In a system such as a microlithographic stepper, the optical beam path can be as large as several meters which would lead to an unsatisfactorily high total amount of losses at that loss rate. For example, an average five meter beam path, even at a transmittance between 99% and 95.5%, as shown for 50-200 milliTorr residual pressures in FIG. 1, corresponds to between a 10% and 37% loss.
It is clear from the above measurement and theoretical fit that one needs to lower the residual pressure of the absorbing species substantially below 100 milliTorr to achieve acceptable optical losses, e.g. less than around 1% per meter of optical path length. Such low pressures can only be obtained using complex and expensive vacuum equipment and/or operating the vacuum equipment for an unacceptably long time. All together, this leads to a substantial and undesirable downtime for pumping and requires complex and expensive equipment. An approach is needed for depleting the beam path of a laser operating below 200 nm, particularly an F.sub.2 laser, of photoabsorbing species without incurring excessive down times or costs.